Large scale applications of high temperature superconductors will require the manufacture of wires or tapes capable of carrying large currents, i.e., such superconductors must have large critical current densities (J.sub.c) sustainable at high magnetic fields. In polycrystalline materials and ceramics, J.sub.c is limited by the formation of weak links at the grain boundaries due to the very small superconducting coherence length and the intrinsically high anisotropy. Another limitation comes from a phenomenon known as a "giant flux creep", a large thermal decay of the supercurrent, which exceeds that in conventional superconductors by orders of magnitude. Hence, in these materials, the pinning of the magnetic flux lines, which determines J.sub.c, is weak. Thus, enhanced pinning of the magnetic flux lines should lead to enhanced values of J.sub.c.
The use of particle irradiation, for example, to introduce additional pinning centers has shown considerable promise. Point defects induced by bombardment with electrons (as reported by Konczykowski, et al in Physia C 162-164, 747 (1989)), fast neutrons (as reported by Sauerzopf, et al in Cryogenics 30, 650 (1990))), or high energy (MeV) protons (as reported by Civale, et al in Phys. Rev. Lett. 65, 1164 (1990)), seem to be effective in raising J.sub.c at low temperatures (4.2K), but they have a much smaller effect on the size of the useful irreversible regime. Irradiation with energetic (GeV) heavy ions, such as Sn, I, Pb, Au or Xe, which produces aligned columnar damage tracks, was shown (by Civale, et al., in Phys. Rev. Lett. 67 648 (1991)) to successfully shift the irreversibility line upward by tens of degrees and to expand the useful regime by several Tesla at high temperatures.
Kumakura et al. in J. Appl. Phys. 72, 800 (1992) report a large effect in tapes of the bismate material Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8 (Bi-2212) after 180 Mev Cu.sup.11+ irradiation and indicate that the damage by Cu might be columnar. A large expansion of the irreversible regime was recently reported by Civale, et al. in Physica C 208, 137 (1993) in Bi-2223 silver-clad tapes modified by irradiation with 1 GeV Au. More recently, T. Hwa et al in Phys. Rev. Lett. 71, 3545 (1993) proposed that a splay (i.e., a dispersion in the orientation of the columnar defects) should lead to an "entangled" state of the vortex matter in which the dissipation caused by the vortex motion via the hopping and spreading processes would be substantially reduced, resulting in a greatly diminished flux creep and increased critical current density.